Angiotensin-II (AII), in addition to being a circulating hormone, is now thought to act as a neuropeptide in the central nervous system (CNS) and may play a modulatory function on the release and subsequent action of other neurotransmitters (Unger et al. (1988) Circulation 77 (suppl I):40-54). Specific receptors for AII with high affinity have been identified and localized in different regions of the CNS (Mann (1982) Exp. Brain Res. 4 (suppl):242). Stimulation of AII receptors in the CNS elicits a complex but very reproducible and concerted pattern of behavioral, cardiovascular, and endocrine responses (Fitzsimons (1980) Rev. Physiol. Biochem. Pharmacol. 87:117). These include CNS-induced elevation of blood pressure, increased drinking and sodium appetite, release of antidiuretic hormone, oxytocin, luteinizing hormone, and prolactin, and other effects (Scholken et al. (1982) Experientia 38:469). The CNS effects of AII could lead to hypertension and other cardiovascular diseases through inhibition of the baroreceptor reflex, increase in salt consumption, volume expansion, and increased peripheral resistance. Besides the cardiovascular system, AII may also influence the reproductive system and other brain functions, such as memory (Koller et al. (1975) Neuroscience Lett. 14:71-75).
The major functions of AII in the CNS can be classified into three groups which may share, at least in part, overlapping mechanisms of action. The first major function of AII in the CNS is regulation of body fluid volume in response to hypovolemia, involving, for example, regulation of thirst, blood pressure increases, vasopressin release, sodium appetite increase, adrenocorticotropic hormone (ACTH) release, and aldosterone release (Unger et al. (1988) Circulation 77 (suppl I):40-54, and references cited therein). This CNS function of AII is closely related to the role of AII in hypertension.
A second function of AII in the CNS, although poorly defined, is the regulation of gonadotrophic hormone releasing hormones and pituitary hormones during the reproductive cycle and pregnancy (Unger et al., supra).
A third possible CNS function of AII is a synaptic function. AII appears to interact with neurotransmitters such as acetylcholine (ACh), catecholamines, serotonin, and other peptides (Unger et al., supra). The amount of data supporting this CNS function of AII is limited. Published results suggest that increased AII activity in brain maintains an inhibitory control on cholinergic neurons resulting in impaired cognitive performance.
The role of peptides in learning and memory was initially investigated by DeWied in the late 1960's and early 1970's, and led Morgan and Routtenberg (Science (1977) 196:87-89) to investigate the role of AII in mediating retention of a passive avoidance (PA) response in rats. These authors demonstrated that rats injected with AII into the dorsal neostriatum, a brain area that has a high concentration of AII as well as precursors and metabolic enzymes for AII biosynthesis, showed a disruption in retention of a PA response. The authors demonstrated specificity of the response in terms of both the location in the brain and the peptide used (thyrotropin releasing hormone or lysine-8-vasopressin had no effect). This study showed that increased AII in the dorsal neostriatum results in a cognitive impairment which is most likely a result of AII modulation of neuronal activity that is necessary for consolidation of newly acquired information.
A different approach for investigating the behavioral effects of AII in the CNS was taken by Koller et al., Neuroscience Letters (1975) 14:71-75. These authors injected renin into the lateral ventricle of the brain (IVT) and measured increases in AII in cerebrospinal fluid (CSF); AII increased from 40 to about 5000 fmol per mL. This increase in AII was accompanied by a disruption of avoidance learning. These results suggested that renin-stimulated biosynthesis of AII could disrupt memory. IVT administration of the angiotensin-converting enzyme (ACE) inhibitors SQ 14225 (captopril), prior to the renin injection, prevented the renin-induced avoidance disruption. We have also shown in our laboratory that renin administered IVT produces a dose-related amnesia in a PA task, which is prevented by IVT administration of the ACE inhibitor captopril. These results suggest that increased AII levels in brain leads to a disruption of avoidance performance. Thus, this amnesia can be achieved by direct application into a discrete brain area of AII or renin, a stimulator of endogenous AII biosynthesis.
In the literature on the neuropathology and neurochemistry of Alzheimer's disease (AD) using human CSF and brain tissue, two reports of altered levels of dipeptidyl carboxypeptidase (angiotensin-converting enzyme, ACE) were published. Arrequi et al., J. Neurochemistry (1982) 38:1490-1492 found increased ACE activity in the hippocampus, parahippocampal gyrus, frontal cortex, and caudate nucleus in AD patients. Zubenko et al., Biol. Psych. (1986) 21:1365-1381, found a correlation between the severity of AD with levels of ACE in CSF. Whether the alterations in ACE are causative in the progression of dementia or correlates of the disease progress is not known.
Recent evidence that inhibition of ACE can have a modulatory effect on learning and memory was reported by Usinger et al., Drug Dev. Research (1988) 14:315-324 (also European Patent Application EP 307,872 to Hoechst, published Mar. 22, 1989). These authors investigated the effects of the ACE inhibitor Hoe 288 on:uphill avoidance in mice, scopolamine-induced (muscarinic receptor blocker) amnesia of a PA response, and a scopolamine-induced impairment of eight arm radial maze performance in rats. In the uphill avoidance test, an acute administration of Hoe 288 at 30 mg/kg PO improved performance during retention testing. In the scopolamine-induced PA amnesia, administration of Hoe 288 three times per day at 1, 3 and 10 mg/kg PO, partially reversed the amnesia. Finally, 3 mg/kg IP partially antagonized the effects of muscarinic receptor blockade on performance. Further, these authors demonstrated that acute or repeated administration of the ACE inhibitor induced a significant decrease in ACh in the striatum and hypothalamus.
Similar results were reported by Costall et al., Pharmacol. Biochem. Behav. (1989) 33:573-579, using the ACE inhibitor captopril. These authors demonstrated that the subchronic treatment with captopril increased the rate of acquisition of light/dark habituation performance. Further, anticholinergic scopolamine-induced disruption of performance in this test model was prevented by daily treatment with captopril.
The ACE inhibitor SQ 29852 has also been reported to provide protective effects on memory of previously learned tasks and to ameliorate, at least in part, an anticholinergic effect on performance (European Patent Application EP 288,907 to Squibb, published 11/2/88).
Evidence for a role of AII on cholinergic function was also reported by Barnes et al. (Brain Research (1989) 491:136-143), who examined the affect of AII on an in vitro model of potassium stimulated release of [3H]ACh. AII, but not AI, reduced potassium-stimulated release of ACh without effects on basal levels. This effect was antagonized by the AII antagonist [1-sarcosine, 8-threonine]angiotensin-II. These results suggest that AII can inhibit the release of ACh in entorhinal cortex from rat brain.
The results summarized above suggest that increased AII activity in brain may maintain inhibitory control of cholinergic neurons, resulting in impaired cognitive performance.